CPU version of crossMapNormalBwd is not implemented

[luotao1]

The CPU version of crossMapNormalBwd is already implemented by @hedaoyuan . Thus, when this function can be merged here?

[backyes]

@luotao1, just add more detailed background descriptions on this issue to follow the suggestions from #283

[luotao1]

crossMapNormalBwd is used in CMRProjectionNormLayer::backward. However, its CPU version is not implemented.

void CpuMatrix::crossMapNormalBwd(Matrix& localGrad, Matrix& denoms,
                                  Matrix& preOutV, Matrix& localOutV,
                                  size_t channels, size_t imgSizeH,
                                  size_t imgSizeW, size_t size, float scale,
                                  float pow) {
  LOG(FATAL) << "Not implemented";

  CHECK(imgSizeH * imgSizeW * channels == preOutV.getWidth());
  CHECK(denoms.getHeight() == preOutV.getHeight() &&
        denoms.getWidth() == preOutV.getWidth() &&
        preOutV.getHeight() == height_ && preOutV.getWidth() == width_);
  CHECK(denoms.getHeight() == localGrad.getHeight() &&
        denoms.getWidth() == localGrad.getWidth());

  // NOLINT // TODO:
}

[hedaoyuan]

OK, I will submit pr, and add to the 0.8.2 version.